Faster VO2 kinetics in arm exercise following circulatory occlusion of the legs

The kinetics of V̇O2 at the onset of sub-ventilatory threshold constant load work might be rate limited by circulatory transport of O2, or by metabolic utilization. To alter the available circulating blood volume relative to muscle mass, 7 subjects performed supine arm cycling exercise at 50W (males) or 40W (females) for 8 min with leg blood flow uncuffed (U) or cuffed (C). Occlusion was obtained in supine rest by rapidly inflating pressure cuffs to 250 Torr. The possibility of a pain response of ventilation and gas exchange was excluded by observing no difference between U and C responses during 12 min of rest or OW cycling. Breath-by-breath gas exchange of six transitions from 4 min OW to 8 min 50 or 40W were superimposed for kinetic analysis by nonlinear regression using a single term exponential with time delay. The results showed no significant effect on the amplitude of the V̇O2 response for OW to work transition (U=504±32 ml O2/min; C=485±35, x̄ ± 1 S.D.). However, the mean response time (time constant + time delay) was significantly (P<0.04) faster for C (66.0±26.4 s) than U (81.2±37.5 s). These data show an increased rate of V̇O2 adaptation in a situation where arm metabolic requirements were maintained constant; presumably arm blood flow was altered. This supports an O2 transport limitation at least at the onset of arm exercise.